1. Field of the Invention
The present invention relates to a coherent optical receiver which is used for terrestrial optical fiber communications, spatial optical communications between terrestrial stations, spatial optical communications between satellites, spatial optical communications between a terrestrial station and a satellite, and communications between fixed and mobile transmitters and receivers. The coherent optical receiver can be synchronized with a clock extracted from an optical phase modulated signal, and compensate propagation distortion of signals, and can receive signals modulated in two optical modulation schemes; i.e., intensity modulation and modulation for coherent communications.
2. Description of the Related Art
There has been known a coherent communication scheme which utilizes laser light's intrinsic nature as a wave, and transmits a signal by modulating the frequency or phase of the laser light, in contrast to ordinary optical communications in which a signal is transmitted by modulating the intensity of light waves. Such a coherent communication scheme can be used in the optical communication field requiring large-capacity transmission, the fiber information communication field, the optical communication field requiring long-distance transmission, the field of space communications near the earth, and the field of deep space communications. In the coherent communication scheme, optical detection (demodulation) is effected by means of mixing signal light and local oscillation light. A phase-diversity reception scheme has been proposed so as to realize such detection of coherent light (see Patent Document 1).
FIG. 4 is a diagram showing an optical receiver of a phase-diversity reception scheme described in Patent Document 1. PSK-modulated input signal light is transmitted from a transmitter (not shown) to the receiver. In the illustrated receiver, the input signal light is demodulated by means of delay detection utilizing a phase-diversity scheme, which is realized by a 90-degree optical hybrid for splitting the input signal light to quadrature components I and Q, a pair of photo detectors, a pair of 1-bit delay lines for delay detection (demodulation), and a pair of two-input, four-quadrant output analog multipliers. The outputs of the multipliers are added together, whereby a signal component is output.
An AFC circuit includes a frequency discriminator, a LPF (low-pass filter) which allows passage therethrough of components near DC which are necessary for AFC, and a local oscillation light source. The output of the local oscillation light source is supplied to the 90-degree optical hybrid. In the phase-diversity reception scheme, the AFC circuit is configured to reduce to zero the frequency difference between the local oscillation light and the signal light.
As described above, the illustrated receiver can demodulate a PSK modulated signal by means of delay detection utilizing the phase-diversity scheme. However, since phase detection is not performed by digital processing, compensation for fiber dispersion and phase compensation for atmospheric fluctuation cannot be performed. Further, the receiver cannot cope with both intensity modulation and coherent modulation.
Non-Patent Document 1 discloses a phase-diversity reception scheme adapted to digital processing. FIG. 5 is a diagram showing a coherent optical reception apparatus of a phase-diversity reception scheme described in Non-Patent Document 1. In the illustrated reception apparatus, signal light transmitted from a transmitter side is input to a phase-diversity homodyne receiver along with local oscillation light from a local oscillation light source. The homodyne receiver mixes the input signal light and the local oscillation light having the same frequency as the input signal light so as to directly obtain low-frequency electric signals IPD1 and IPD2. The low-frequency electric signals IPD1 and IPD2 have a phase difference of 90° therebetween (SIN and COS waves), and carry pieces of information regarding the amplitude and phase of the optical signal, respectively. The low-frequency electric signals IPD1 and IPD2 are led to a digital processing circuit DSP via respective low-pass filters LPF and analog/digital converters ADC. The digital processing circuit DSP detects a carrier phase, and demodulates data. Normally, the frequency of the local oscillation light must be made coincident with that of the input signal light by use of PLL (phase-locked loop: phase synchronization circuit). In contrast, the illustrated coherent optical reception apparatus does not utilize PLL and absorbs the frequency deviation through the digital processing. However, in reality, achieving transmission speed of a few Gbps in real time is difficult until new devices are developed. Since the reception apparatus shown in FIG. 5 does not employ PLL, a beat attributable to the frequency deviation is superimposed on the output of the homodyne receiver. Therefore, sampling at the analog/digital converters ADC cannot be performed in synchronism with the data signal such that sampling is performed one time in each symbol period. Therefore, when a signal is transmitted at, for example, 10 Gbps, the analog/digital converters ADC must perform high-speed processing of about 100 G samples/S, which is ten times the transmission speed (see FIG. 3).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 11-41207
[Non-Patent Document 1] Kazuro Kikuchi, “Phase-Diversity Homodyne Detection of Multilevel Optical Modulation With Digital Carrier Phase Estimation,” IEEE JOURNAL OF SELECTED TOPICS OF QUANTUM ELECTRONICS, VOL. 12, NO. 4, JULY/AUGUST 2006